The present invention relates generally to flood sources used in imaging systems. Specifically, the present invention is an improved flood source, and method of making the same, which emits an evenly distributed flow of energy from a gamma emitting radionuclide dispersed throughout the volume of the flood source.
The use of imaging systems that utilize energy from a gamma emitting radionuclides is a well known and well understood technique. The technique relies in one aspect upon those sources of energy, which are known in the trade as flood sources. Flood sources are typically used for checking the performance of nuclear medicine imaging systems. The size and shape of the flood sources are designed to closely match the size and shape of the gamma camera head. When this flood source is placed in the field of view of a nuclear medicine gamma camera, the flood ideally presents a uniform level of activity that produces a uniform gray image in the camera field-of-view. Defects in the image then appear as lighter or darker regions that indicate the locations of camera detectors that are not properly tuned. Thus, it is highly desirable to have the flood source present as uniform a level of activity as can be achieved.
Several investigators have attempted to solve the problems created by flood sources with less than optimum uniformity. For example, U.S. Pat. No. 4,882,494, issued Nov. 21, 1989 to Rogers, et. al. and entitled xe2x80x9cApparatus and method for flooding a nuclear imaging device with radiation from an imaging sourcexe2x80x9d describes a method for correcting the non-uniformity in count density of nuclear imaging sources and an apparatus for carrying out the method. As described within the patent, the problem arises because standards with a stated certified non-uniformity of xc2x11% may actually have a much higher non-uniformity if standard National Electrical Manufactures Association (xe2x80x9cNEMAxe2x80x9d) procedures are applied, since certified non-uniformity is based on a sampling of counts from the surface of the standard rather than on a total surface radiography. Also, since non-uniformity is quantified from counts in only two smoothed pixels (the maximum and the minimum), the method is potentially dependent on small flaws. Thus, to solve these problems the ""494 patent proposes a method which includes the steps of positioning a nuclear imaging flood source beneath or adjacent a nuclear imaging device such as a gamma camera having a plurality of radiation sensing elements and moving the source in a controlled motion thereby blurring or diminishing the effects of any non-uniformities in the source, (i.e., insuring that each sensor or detector in the camera receives radiation from more than one radiation-emitting area of the source so that each sensor receives a more similar amount of light (radiation) than if the source was not moved). As taught by the ""494 patent, the movement of the source can be accomplished over a long period of time, in a pattern, in a reproducible pattern, continuously, in a complex motion, or repetitively. Clearly, if the problems solved by such a method may be solved instead simply providing a uniform flood source, the type of expensive and elaborate equipment required by schemes such as that taught in the ""494 patent may be eliminated. Thus, there exists a continued need for more uniform flood sources.
Several different designs of flood sources are available from a variety of commercial suppliers, and it may be easily appreciated that the market for flood sources is keenly competitive. As such, one may readily survey the state of the art in flood source design and manufacture simply by viewing the competitive landscape. For example, the Syncor(copyright) Corporation of 6464 Canoga Avenue, Woodlad Hills, Calif. currently distributes a variety of rectangular and circular flood sources made by DuPont that are claimed to have a coefficient of variation of less than one percent (1%) with less than three percent (3%) maximum deviation from the mean, a differential non-uniformity of less than one point eight percent (1.8%), and integral non-uniformity of less than two point five percent (2.5%). The Biodex Medical Systems Corporation of 20 Ramsay Road, Shirley, N.Y. 11967-0702 makes available Co57 flood sources which it advertises as having non-uniformity of less than plus or minus one percent (1%) at two standard deviations. Thus, while the state of the art of flood source manufacture has produced flood sources having high levels of uniformity, there exists a need for improved methods for making flood sources and improved flood sources having even greater levels of Co57 source uniformity.
Accordingly, it is an object of the present invention to provide a flood source that presents a uniform radionuclide source distribution and emits energy having a high level of uniformity across the surface of the flood source.
It is a further object of the invention to provide a flood source placed within a shell having a bottom and sides defining a volume, wherein the flood source is made of a batch consisting of an epoxy resin of CY-507 resin and HY-2963 hardener in a ratio of resin to hardener from 2.5:1 to 3:1, and the epoxy resin is further mixed with a gamma emitting radionuclide having an energy between 90 keV and 375 keV, and at least one colored dye that provides a visual indicator of uniformity of the flood source batch and a cover fitting over the shell.
It is a further object of the invention to provide a flood source wherein the gamma emitting radionuclide is Co57.
It is a further object of the invention to provide a flood source wherein the gamma emitting radionuclide is Co57 and is distributed uniformly throughout the volume of the flood source batch.
It is a further object of the invention to provide a flood source wherein the gamma emitting radionuclide is Co57 emits at least 5 millicuries.
It is a further object of the invention to provide a flood source wherein the gamma emitting radionuclide is provided in the chemical form CoCl2.
It is a further object of the invention to provide a flood source wherein the shell and the cover are fabricated from ABS plastic.
It is a further object of the invention to provide a method of fabricating a flood source having the steps of providing a shell having a bottom and sides defining a volume, preparing a mixture of HY-2963 hardener with a gamma emitting radionuclide having an energy between 90 keV and 375 keV and at least one colored dye that provides a visual indicator of uniformity of the flood source batch, mixing the hardener, radionuclide, and colored dye until the colored dye is uniformly dispersed throughout said hardener, mixing the hardener, radionuclide, and color dye with an epoxy resin of CY-507 resin in a ratio of resin to hardener from 2.5:1 to 3:1 until the colored dye is uniformly dispersed throughout the epoxy resin, placing the mixture of hardener, radionuclide, color dye, and epoxy resin within the volume defined by the shell, allowing the mixture to cure, and sealing the shell with a cover fitting over the shell.
It is a further object of the invention to provide a method of fabricating a flood source wherein the shell is affixed to a level surface when the mixture of hardener, radionuclide, color dye, and epoxy resin is cured to prevent the shell from warping due to the heat of the curing reaction.
It is a further object of the invention to provide a method of fabricating a flood source wherein the shell is affixed to a level surface by means of a heat conductive clamp when the mixture of hardener, radionuclide, color dye, and epoxy resin is cured within the volume defined by the shell to prevent the shell from warping due to the heat of the curing reaction.
It is a further object of the present invention to provide a heat conductive clamp fabricated of aluminum.
It is a further object of the invention to provide a method of fabricating a flood source wherein the mixture of hardener, radionuclide, color dye, and epoxy resin is purged of bubbles prior to curing the mixture within the volume defined by the shell.
These and other objects of the invention are accomplished through the present invention which provides an improved design for flood shells featuring a durable and attractive vacuum-formed ABS plastic outer shell, and a durable and uniform epoxy resin filler of uniform thickness and which contains a uniform distribution of the radionuclide cobalt-57. When this flood source is placed in the field of view of a nuclear medicine gamma camera, the flood presents a uniform level of activity that produces a uniform gray image in the camera field-of-view. Defects in the image appear as lighter or darker regions that indicate the locations of camera detectors that are not properly tuned. Typically, the active volumes of the flood sources are approximately 24-inches round by ⅙ to ⅕-inch thick for a round flood source, or 16xc3x9724-inch by ⅙ to ⅕-inch thick for a rectangular flood source, although the present invention contemplates a wide variety of shapes and sizes for flood sources and the invention should be understood to encompass any flood source manufactured by the method described herein, regardless of its shape or size. The flood sources of the present invention are preferably formed with sturdy handles for carrying and positioning under the camera. Preferably, a lead-shielded shipping case is used to transport and store the flood source when it is not in use. The physical half-life of the cobalt-57 activity in the flood source of the present invention is approximately 271 days (9 months), meaning that the activity within the flood source decreases by one-half every nine months.